1. Field of the Invention
The present invention relates to the technical field of measuring light sources, specifically, to the method for measuring the luminous flux of LED.
2. Description of the Related Art
Light Emitting Diode (LED) has the features such as small volume, long lifetime, being driven by low voltage D.C. and excellent monochromaticity; it can be widely applied to the fields of displaying device and special illumination. In theory, LED has high light-emitting efficiency, up to 350˜370 lm/W in white LED, for example; therefore, it has a good application prospect in the general illumination market. Among various LED applications, the external quantum efficiency, that is, the ratio of the flux of emitted luminous flux to the input electric power, is an important index for LED. Measuring the external quantum efficiency of LED is difficult in measuring the luminous flux. Owing to the features of LED, up to now, there is no generally-acknowledged and simple method similar to the traditional one for measuring the luminous flux of light sources in the world. There are two methods for measuring the total luminous flux of light sources, namely, the absolute method and the relative method. In the absolute method, the goniophotometer and the like are used to measure the angular intensity distribution of a light source, and the total luminous flux of the light source is calculated based on the distribution. This method can be applied to all kinds of light sources and may achieve high accuracy in principle, but both measurement and calculation are time-and-effort-consuming processes. In the relative method, the total luminous flux is achieved by comparing the light source to be measured with a standard light source whose total luminous flux is known, and the measuring instruments commonly used are an integrating sphere and an illuminometer. Both measurement and calculation of the relative method are simple and practical, so this method is widely applied. However, LED is a semiconductor light source, and its light-emitting mechanism is different from the traditional light source and has its particularity. The following are the problems which will take place when the traditional method using integrating spheroid is used to measure the luminous flux of LED.    (1) The sensitivity of the detector R (λ) does not match the spectral luminous efficiency function V(λ) of human eye
In general, the photometer is calibrated by using an incandescent lamp, the average value of the mismatch error (refer as SCF) is evaluated within the range of whole visible lights, and the average error within the range of whole visible lights is defined when the evaluation is carried out. Because the absolute values of V (λ) in the blue wavelength and the red wavelength are very small, the percentage of the error in these wavelengths is much higher than SCF, and the photometer well calibrated in SCF is applicable to measure the light source which emits a continuous spectrum. Because the bandwidth of LED is only 20 nm to 40 nm typically and the peak wavelengths of LEDs of various colors cover the range of visible lights and the adjacent ranges thereof, significant error is still inevitable even if the photometer well calibrated in SCF is used to measure the luminous flux of blue (red) LED.    (2) self-absorption effect of the object in the integrating sphere
In the traditional method using integrating sphere, some objects like the light source to be measured, a baffle, a holder and the like should be placed in the integrating sphere. LED is not only a pure light source, but also an object capable of reflecting and absorbing the light in the sphere, therefore, LED may absorb a portion of light in the sphere and result in the negative error of measurement; the baffle and the holder also inevitably lead to changes in the distribution of light in the sphere, so the theory of integrating spheroid can not be satisfied and principle error of measurement may occur. Because the integrating sphere used to measure LED is typically very small, for example, only 5 cm in diameter, the influence of the self-absorption effect of these objects cannot be ignored.    (3) heat sink
LED is a temperature-sensitive device, the total output luminous flux of which will rapidly decrease as the temperature of the LED rises. Therefore, in order to ensure the stable output luminous flux, it is necessary to sufficiently dissipate the heat from LED in operation, but if LED is placed in the center of the integrating sphere for measurement, the temperature of LED will be raised because the need for heat sink can not be satisfied in the hermetic space inside the integrating sphere, resulting in the unstable light output from LED and the error of the measurement.
Owing to existence of above-mentioned problems, in the current measurement of luminous flux of LED, there are problems such as poor reproducibility, large indefiniteness in measurement, poor consistency in measuring results between different measuring devices and the like.
The problems existing in the measurement of luminous flux of LED are followed with interest from the International Illumination Committee and the industrial associations of various countries. The publication CIE 127-1997 “Measurement of LEDs” made some discussion about the measurement of luminous flux of LED and advanced several integrating sphere devices for measuring the luminous flux. In each of these measuring devices, a traditional integrating sphere is used and LED is placed on the surface of the sphere, but a baffle has to be placed inside the sphere in order to shadow light from LED directly to the detector. When the baffle is placed, since the theory of integrating block light from LED directly to the detector cannot be fully satisfied, especially when the integrating sphere is smaller, larger errors will be caused. Therefore, only a standard LED whose distribution is similar to that of the light source to be measured can be used as the standard light source, but this standard LED is very difficult to achieve because of the diverse shapes of light output from LED; at the same time, the degree of difficulty in the measurement is increased.
Recently, in several intermediate conferences of the related technical committee of CIE, the methods for measuring the luminous flux of LED are discussed, but the standard for measuring the luminous flux of LED remains in research all the way. The scientists of the National Institute of Science and Technology (NIST) of the U.S. have made more meticulous researches over the method using integrating sphere for measuring the luminous flux of LED, especially in detail the influences of the baffle inside the integrating sphere and the self-absorption of light source upon the measuring results. It is discovered that the self-absorption will exert a certain influence in the case of a small-sized integrating sphere. At the same time, the influence of the accuracy with which the detector is matched with V (λ) upon the accuracy with which the luminous flux of LED is measured has been also been researched in detail.
The Institute for National Standard of Canada has researched in very detail the method using integrating sphere for measuring LED. In these researches, the method of ray simulation is used to research the difference in the measurements with integrating sphere of the beamed light of LED, and to analyze the influences of the baffle inside the integrating sphere, the self-absorption of light source and the like.
The National Institute of Metrology of P. R. China has made a lot of researches on measuring LED and established the photometric standard of LED at the national level, which fills in the blank in the field of LED photometric measuring in a certain degree. The researchers of Zhejiang University and the Yuanfang Measuring Technology Company in Hangzhou have made a lot of researches on measuring LED and advanced the method for measuring the luminous flux through LED spectrum correction, as well as researched the subject of self-absorption during the measurement of the luminous flux of LED.
Recently, Fudan University has also made a lot of researches on measuring LED and advanced a method using integrating sphere that adopts a narrow beam standard light source, and in this method, a light source to be measured and a standard light source are placed on the surface of the integrating sphere, and the relative spectrum distribution of LED is measured with CCD spectrometer and the value of the luminous flux is therefore calculated. Although this method can eliminate the influence of the matching error of V (λ) upon the measuring results and relieve the influence of the heat sinking in a certain degree, yet because the integrating sphere for measuring the luminous flux of LED is typically small-sized and at least two holes should be opened thereon, one for placing LED to be measured and the other for the fiber interface, the integrating sphere will be deviated from the ideal integrating sphere and therefore the measuring errors will still exist.
To sum up the current researches on measuring the luminous flux of LED, it is discovered that these researches are still using the traditional methods with integrating sphere for measuring the light source; while larger errors exist in the measurement of the luminous flux of LED using the integrating sphere, these methods can not form a standard and be popularized. According to the features of LED, that is, LED is small in size and emits light in a solid angle of 2π, the present invention advances a novel measuring method, in which a reflecting cup is used as a collector to concentrate the light emitted from LED, and then the luminous flux thereof is measured by a detector.